Electrical system and method of establishing same



Oct. 5, 1965, 7 -p. NELSON 3,210,604

ELECTRICAL SYSTEM AND METHOD OF ESTABLISHING SAME Filed Aug. 27, 1962 2 Sheets-Sheet 1 IN VEN TOR.

Gal/am! 0Z9. V4501 attorney;

Oct. 5, 1965 Filed Aug. 27, 1962 R. D. NELSON 3,210,604

ELECTRICAL SYSTEM AND METHOD OF ESTABLISHING SAME 2 Sheets-Sheet 2 LOG IME) X Y A 8 T I I L I AREA \II/HEREIN ONE FUSE 24 54 OPENS OIRcUIT MELTING TIME OF: \F OLEARINO 2;

HID/55022 54 FUSE 65 I? 2I 5I FUSE 2|,5l

\ AREA \II/HEREIN ALL TRANSFORMER FUSES 5 2| 5| S 24 54 OPEN I OIROUIT 1 7 MINIMUM CLEARING TIME 0.8 CYCLE 1 LOG IOURRENT) INVENTOR. F/ 4 Y Qoiizz'rrdfl. W660 m/cf. Qatar??? United States Patent 3,210,604 ELECTRICAL SYSTEM AND METHOD OF ESTABLISHING SAME Rolland D. Nelson, Hales Corners, Wis., assignor to RT & E Corporation, Waukesha, Wis. Filed Aug. 27, 1962, Ser. No. 219,514 4 Claims. (Cl. 31715) The present invention relates to an improved electrical fuse system and method of establishing the same. One aspect of the invention relates to an electrical fusing system comprising a plurality of fusing elements of different time-current characteristics so arranged that a preselected one, or ones, of the fusing elements blow responsive to faults in, respectively, the power and load side, or both, of the associated electrical circuit. Another aspect of the invention concerns an electrical fusing system adapted for use with encased electrical apparatus. The present invention is particularly adapted for use with encased electrical apparatus wherein service conditions dictate fuse replacement with maximum safety and efiiciency.

The invention is especially suited for use in an encased oil-cooled distribution transformer, in connection with which the invention will be described by way of example but not as a limitation.

It is known to provide in distribution transformers a fuse on the primary side (i.e. the source side) to isolate the system from the transformer. The prior art knows too, of providing an additional fuse of lower capacity on the secondary side (i.e. the load side) to protect the transformer against overloading. The prior art has also used circuit breakers or other disconnect means as a substitute for either or both of the secondary side or primary side fuses.

It is also known that fuses of different structures each have a different time-current characteristic and that the environmental conditions to which fuses are subjected change their time-current characteristics. It is also known that certain classes of fuses have an upper limit on the current they can safely handle and that current exceeding this upper limit may cause the fuse to explode or violently disintegrate. The practice has been to match the secondary side fuse with the primary side fuse with respect to the time-current characteristics of each. The secondary side'fuse has generally been selected to open the circuit first upon load side faults or overloads.

Faults on the primary side within the transformer are ordinarily of a character such that the transformer involved should be removed from service, replaced with another transformer, and itself be subjected to disassembly, close inspection and any appropriate rebuilding prior to being returned to service. Hence, fuses in the primary side circuit have customarily been placed in a relatively inaccessible location within the transformer to insure adequate servicing before returning the unit to service.

Faults and overloads on the load side are oftentimes of short term nature and otherwise of such character that the transformer can remain in service provided, of course, that a new fuse is installed. Secondary side fuses have in the prior art been disposed externally in the circuit on the load side and secondary side breakers have in the prior art been disposed internally of the transformer between the secondary windings and the load side connections.

Unfortunately it is often difficult if not impossible to determine the character of the fault which has caused a transformer fuse to blow. Special accessory equipment of comparatively expensive, and often complicated, nature has often been combined with power distribution transformers to enhance the safety of servicing them.

It is therefore an object of the invention to provide in a transformer an improved electrical fuse system having enhanced safety characteristics.

3,210,604 ?a.tentecl Oct. 5, 1965 The time-current capacity of the primary side transformer fuse has been used as the base upon which to select the sizes of other fuses in the power distribution systems between the transformer and the power generator, giving due consideration to other factors such as response time, wire size and total current drawn by other loads and transformers connected in parallel with the transformer being considered. This has led to fuses of larger than necessary time-current capacities being selected for the system. Oftentimes the system fuse capacity is improperly sized in either or both of time response or current rating because the basethe primary side fuse in a given transformer-starts out at too large a size.

One object of the present invention is to provide a fus ing arrangement for the primary side of a transformer whereby safety is enhanced while also allowing a more realistic and economical sizing of fuses elsewhere in the distribution system with which the transformer is associated.

Another object of the invention is the provision of a transformer fusing arrangement whereby there is realized an improved sizing of fuses in the system with which the transformer is associated.

An object of the present invention is to provide an improved electrical fuse system and a method of establishing the same.

It is a further object of the present invention to provide for multiple primary side fusing of a transformer that will distinguish between external load overloads or faults and internal transformer faults.

It is a further object of the present invention to provide a convenient load making or breaking means for connecting the transformer to the distribution circuit and yet not jeopardize the safety of the operator.

Another object of the present invention is to provide for primary side fusing that will fail safe and prevent refusing following internal fault conditions of the apparatus.

Still another object of the present invention resides in providing a fuse system for protecting the distribution system concerned from faults internal to the apparatus.

This invention comprehends a fusing system for an encased electrical apparatus wherein the fusing system includes at least two fuses connected in series with each other. One of the fuses is inaccessibly disposed inside the apparatus casing or housing. The inaccessible fuse preferably has a time-current rating larger than the second one of the fuses which latter is disposed for access from outside the casing.

Preferably the fuses are combined with the primary side of a transformer, being connected in series therewith. Each of the fuses in the pair is advantageously of predetermined time-current characteristics such that the second fuse which is removable will blow responsive to source side conditions arising from an overload or a fault on the load side, while the first-mentioned fuse which is inaccessible will also open at higher current values. Preferably both fuses are matched (as seen on a graph of current-time characteristics) to have crossing time-current curves for a current value in excess of the current obtained under maximum load short circuit conditions.

The second fuse is preferably of the removable type and accessible from the outside for replacement from outside the transformer casing and may advantageously be constructed in accordance with United States Patent 2,918,557, issued December 22, 1959, for a Circuit Breaker. The isolating or first fuse is preferably of nonremovable character and is inaccessibly disposed inside the transformer casing whereby servicing of the same ordinarily dictates removal of the transformer concerned from service.

The curve-crossing feature of the invention contem- 3 plates matching a small fuse with a large fuse so that at a predetermined current, the second or smaller fuse clears (i.e. melts and then extinguishes any arc) in the same length of time that the first or larger fuse melts. In this fashion the total clearing time of the first fuse responsive to such current or a larger current is ostensibly reduced because of the interruption of the current by the second fuse; the second or smaller fuse clears (stops arcing) While the first or larger fuse is still arcing, and being a series circuit, the arc of the first fuse is extinguished simultaneously. The predetermined current is preferably greater than that drawn by a full load side short circuit.

A feature of the invention is the manner of coordinating the fuses according to their respective characteristics. A further advantage of the present invention is that its cost is considerably less than other systems directed to the same or related problems. For example, based on present day costs, the present system costs from about 30% to 70% less than a system employing a load side circuit breaker and a source side fuse.

' The disposition of the first or isolating fuse in an inaccessible location such as the inside ef the transformeroffers several advantages. One is that the transformer would normally be taken out of service anyway should a fault occur on the primary side, and such a fusing arrangement makes this mandatory. Further, it is extremely difficult, if not impossible, to replace this fuse until after such removal from service, hence the chances are remote of carelessly replacing this fuse and creating a dangerous apparatus condition.

Other advantages, features and objects of the invention will become apparent from the following description when read in conjunction with the annexed drawings wherein:

FIG. 1 is a schematic elevation view of one embodiment of the invention as applied to a single phase oil cooled transformed for connection to an ungrounded system.

FIG. 2 is a schematic elevation view of another embodiment of the invention as applied to a single phase oil cooled transformer for connection to a grounded system.

FIG. 3 is a representative single line schematic of a distribution system from a generator through 'a series of fuses to a plurality of loads.

FIG. 4 is a representative composite idealized timecurrent characteristic on log-log coordinates for the two series fuses showing the different blowing characteristics that make them differentiate in type of load and fault.

Referring now to FIG. 1, there is seen a' transformer assembly 1 comprising a casing assembly 3 containing a body of oil 5 in which there is immersed a conventional transformer coil and core assembly '7. A source side coil 8 is a portion of the transformer assembly. The source side coil is the primary coil and would be on the high voltage side in a step down transformer. Similarly, the source side coil 8 would be on the low voltage side in a step up transformer.

Leads 9 and 10 extend from opposite ends of the primary c'oil through the body of oil and pass out through the removable lid 13 of the casing assembly by means of the pair of like-constructed bushings 11, 12. The lid 13 is removably attached to the remainder of the casing by any suitable means such as studs or wing nuts or the like.

The transformer assembly 7 includes a secondary coil 14, a core 15, and secondary leads 16 connected to conventional lead terminations 17, the latter being above the oil.

Fuse 21 and fuse assembly 22 are disposed in series with each other and in series with the coil 8. In the ungrounded system single phase transformer represented in FIG. 1, fuse 21 is immersed in oil 5 and disposed in lead 9 between the coil 8 and the bushing 11. The second fuse assembly 22 is immersed in oil 5 and is disposed in the lead 10 between the coil and the bushing 12.

The first fuse 21 is of conventional construction, is permanently connected in the lead 9, and is non-removable from its circuit except upon removal of the lid 13. That is, fuse 21 is normally inaccessible from outside the transformer casing whereby replacement thereof is achieved only by disassembling the casing as by removing lid 13 to provide access to fuse 21. Fuse 21 is a single element type of fuse larger in time-current rating than a second fuse 24 mounted in assembly 22.

Fuse assembly 22 is a removable externally accessible fuse assembly having a second fuse 24 disposed under oil. It is the second fuse 24 which is connected in series with the coil 8. The fuse assembly 22 is preferably eonstructed in accordance with United States Patent 3,014,158 and/or Patent 2,918,557. Fuse 22 is oftentimes called a Bayonet fuse because of its being externally removable by means of the disengageable locking handle 25. Second fuse 24 is preferably one size smaller in time-current rating than fuse 21. Second fuse 24 functions by blowing and is a means for protecting the load side by responding to overloads by disconnecting the source side elements protected (here the coil 8). Second fuse 24 will blow 0r disconnect responsive to overloads and secondary short circuits. Second fuse 24 disconnects or blows before fuse 21 because fuse 24 is Smaller in rating and the same current flows through each fuse.

Fuse '21, on the other hand, will blow in response to primary winding faults (fuse 24 will also blow) and under circumstances when the transformer must be removed fromservice. In such circumstances the blowing of use 21' will cause no hardship because the transformer will have to be thoroughly inspected and if necessary repaired or rebuilt before being returned to service. The repair, inspection, rebuilding and so forth all require gaining access to the inside of the casing 3 and disassembling the transformer in whole or in part, which would be required anyway in order to replace the inaecessibly located fuse 21. v

Fuse assembly 22 is removed via handle 25 from outside the casing and the second fuse 24 replaced in order to restore the transformer to service. Service on the sec ondary or load side of the transformer will be restored in all cases,- as the general rule, except when the transformer has a permanent internal fault, fuse 21 has blown, or the secondary system remains faulted.

It is to be stressed that the fuse assembly 22' and its second fuse 24 are sized on a time-current basis and arranged to protect the load side of the transformer, but being smaller in rating and disposed in series with the first or isolating fuse 21, will ordinarily also disconnect responsive to internal faults of large magnitude. Heretofore, secondary-protecting disconnect means, either fuses or circuit breakers, have been disposed in the secondary or load side circuit. Circuit breakers have one serious drawback wherever disposed because of their limitation in response time--the mechanical elements asymptoti= cally approach a lower limit of time response which apparently cannot be improved upon for any particular breaker. Fuses, lacking moving parts, for this reason can respond much more rapidly in carrying out their disconnect functions.

The invention comprehends that additional fusing may be employed in conjunction with a system such as illustrated in FIG. 1. Preferably, such additional fusing should constitute a fuse identical with the fuse 21 inserted in series with fuse element 24 on either side of the latter. The additional fuse should, of course, be non-accessibly disposed within the casing.

FIG. 2 represents a transformer having one side of the primary coil grounded. In FIG. 2 the transformer assembly includes a casing assembly 33 surrounding a body of oil 35 and a transformer core and coil assembly 37. The primary coil 38 has a lead 39 connected to ground by an appropriate terminal means 42 connected to the grounded casing. Another lead 4t) extends from the other terminal of the coil 38 through the bushing 44 on the lid of the casing assembly. The disconnect means constituting the first fuse 51, the fuse assembly 52, and the second fuse 54 which is a portion of assembly 52, are connected in series with each other between the coil 38 and the connection to lead 40 of the bushing 44. The first fuse 51 and second fuse 54 are disposed in the body of oil 35. The fuse 51 and second fuse 54 may be interchanged in position and still be effective.

The FIG. 2 transformer assembly 37 is completed by conventional apparatus including a core 45, a secondary Winding 46, and lead terminations 47 to each of which is connected one of the secondary winding leads 48.

Second fuse 54 is, like second fuse 24, a means for protecting the load side responsive to load side conditions as sensed on the source side by disconnecting the source side circuit.- Th second fuse 52, including the second fuse 54, is externally removable and is constructed substantially the same as fuse assembly 22 and second fuse 24 as described above. The ratings second fuse 54 is load breaking and is one size smaller in time-current than first fuse 51. Second fuse 54 may also be a special element of the overload sensing type, as can the second fuse 24. The fuse assemblies 22, 52 and their respective fuses 24, 54 are removably connected in their respective circuits.

Fuse 51 is non-removably and inaccessibly disposed within the casing. By non-removably and inaccessibly disposed is meant the necessity of gaining access to the fuse by removing the cover 41 which is attached to the remainder of the casing in the same fashion as described for the lid 13. The fuse 51 is preferably a single element type of fuse larger in time-current rating than the fuse element 54.

The second fuse 54 will blow or disconnect in response to overloads and secondary short circuits before fuse 51 will blow because the latter is slower and the same current flows through each under different overcurrent conditions.

Fuse 51 will blow on severe primary faults. Due to the arcing time in fuse element 54 the current persisting sufficiently long to melt fuse 51 and under circumstances when the transformer ought to be removed from service anyway. Hence, the blowing of fuse 51 will cause no hardship.

Replacement of fuse element 54 restores service in all cases except where the transformer has a permanent internal fault, fuse 51 has blown, or the secondary system remains faulted. In addition, if the transformer has been subjected to severe primary faults, replacing of the fuse element 54 will not restore service because the fuse 51 must be replaced. To replace fuse 51, it will be recalled, the transformer must be removed from service or disassembled as described in connection with FIG. 1. The latter feature will remove the danger of throwing the fuse 54 in a solidly grounded fault because fuse 51 has blown.

The cooperation between first fuse 21, 51 and the fuse assemblies 22, 52 (and necessarily the corresponding second fuses 24, 54) may be understood by reference to the mechanism of fuse blowing. When a fuse blows it initially goes through a melting phase where there is still a conducting means (a metallic body undergoing melting) between the terminals connected by the fuse. After the melting phase is completed, and an electrical gap appears in the circuit, arcing takes place between the terminals connected by the fuse. The arcing continues until suppressed. In the present embodiment, arcing continues until the alternating current goes through zero whereupon the arc desists or is attenuated and is smothered by the oil in which the fuses or fuse elements are disposed as explained above. The time required for melting and for the extinction of the arc is called the clearing time. The melting, arcing, and clearing times are dependent upon the magnitude of the current and voltage impressed on any given fuse.

Referring now to FIG. 4 and bearing in mind the above blowing hypothesis, assume for a moment that the curves X and Y respectively represent the total melting and clearing characteristics of the smaller or second fuses 24, 54 and the melting characteristics of the larger or first fuses 21, 51. Both fuses have a common time-current value designated 58. The valves plotted above value 58 determine for the present terminology which fuse is smaller by reason of the time required for a given current value to the left of 58 to cause the fuse to blow. Thus, curve X for fuse 24 (54) represents the smaller fuse, Y, the larger fuse 21 (51).

It will be observed from FIG. 4 that curve X comprises two separate fuse characteristics: the melting characteristic being designated by a dotted line and the clearing characteristic being designated by a solid line. The dotted curve X represents at various currents the melting time of the smaller or second fuses 24, 54. The solid curve X represents at various currents the clearing time of the second fuses 24, 54. The difference in time between the dotted and solid line curve X represents the arcing time interval at various current loads.

It will also be observed from FIG. 4 that curve Y comprises two separate fuse characteristics: the melting characteristic being designated by a dotted line and the clearing characteristic being designated by a solid line. The dotted curve Y represents at various currents the melting time of the second fuses 24, 54. The solid curve Y represents at various currents the clearing time of the second fuses 24, 54. The difference in time between the dotted and solid line curve Y represents the arcing time interval at various current loads.

The clearing time characteristics departs substantially from the melting time characteristic as the current impressed on the particular fuse increases. The arcing time increases as the current increases, generally speaking. Also, the curves demonstrate that the clearing time and melting time decrease as the current flowing increases. This is true of each of the fuses represented respectively by curves X and Y. The remaining curves A and B represent melting times only, it being understood that their clearing time curves are similar to those shown for curves X and Y.

The invention takes advantage of the departure of the clearing time curve from the melting time curve. In this fashion, the curves X and Y cross so that clearing time for the smaller fuse 24, 54 (curve X) coincides at point 58 with the melting time for the larger fuse 21, 51. This achieves the ostensible acceleration of larger fuse clearing time because the smaller fuse clears the circuit, stops the flow of current, and therefore extinguishes any arc in the larger fuse on those occasions when the current flowing is greater than that designated by the point 58. In other words, above a certain current value (designated as point 58) the larger fuse has a melting characteristic which resides in the arcing time zone of the smaller fuse. At currents smaller than that designated by the coincident point 58, only the smaller fuse will respond to current flow and go through the blowing sequence, leaving the larger fuse undisturbed.

Preferably, the current value for the point 58 is selected to be equal to, or in some instances, greater than, a current value equal to that current which would flow in the source side due to a full short circuit at the terminals of the load side.

Stated another way, the fuses represented by curves X and Y are matched so that their arcing times overlap for currents above a predetermined value, whereby at a given current, time of melting and arcing of second fuse is sufficient to melt the first fuse but at a lower current is insufficient time to melt said first fuse.

. The curves of FIG. 4 are representative only and it is to be kept in mind that the curve shapes may be varied considerably by changes in fuse alloys and in the fuse environment. Fuses of entirely different alloys may be used together. Fuses when matched according to the invention may have their time-current characteristics altered by the cooling to which they are subjected: e.g., a fusecan be made to blow sooner at low currents by insulating it froma cooling medium. If desired, fuses can be so matched that their melting curves cross. The invention contemplates all of these embodiments as methods to make the time-current characteristics cross or meet at a predetermined point,- FIG. 4 being used for illustrative and explanatory purposes. 7 I

The cooperative disconnect or blowing takes place, assuming a condition on the load side drawing a current designated 57, by the fuse 24, 54 blowing in the manner mentioned above. This condition obtaining and fuse 24, 54 blowing, nothing happens to the larger fuse 21, 51 on curve Y because the series circuit (FIGS. 1, 2) is opened beforeenough time has elapsed to melt the larger fuse 21, 51. The transformer may be restored to service by replacing the smaller fuse 24, 54 by means of the convenient removable externally accessible fuse assembly 22, 52. Repeated blowing of fuse 24, 54 indicates trouble in the load side which, when repaired, allows restoration of service by the means herein described. y p 7 However, a larger current 59 causes both fuses to blow with the smaller fuse 24, 54 melting first and, while going through the melting and arcingphases, sustains the current long enough to cause the larger fuse 21, 51 to melt and also open the circuit. With both fuses blown, the replacement of small fuses 24, 54 does not return the transformer to service. In effect, once fuse 2 1, 51 blows, the rest of the distribution system is isolated or insulated from the transformer concerned. The transformer is removed from service and serviced as required, being replaced with another unit. Higher currents than full load short circuit current 58, result only if internal faults and damage to the transformer haveoccurred and fuse 21, 51 is sized to melt and open only in combination with fuse 24, 54 and such higher currents. Fuse 2 1, 51 blowing indicates internal damage to the transformerand need for replacement.

A beneficial result, alluded to above, in terms of smaller fusing in the distribution system, flows from practice of the invention. FIG. 3 represents a typical distribution system wherein a generator 56 transmits-the required voltage and current to a plurality of loads 60. The electrical power forthe loads is carried by a plurality of serially connected fuses designated 65; 66 and 67 going from the generator to the load.

In the prior art, each of the load transformers 64 is provided with a fusing means 65 on its source side. The fusing means 65 may be of various sizes according to the loads carried by their respective transformers. A branch fuse 66 protects the rest of the system from faults occurring in the line between it and the transformer fuse 65. In practice the fuse 66 is sized above the larger one of the fuses 65.

The present invention employs the cooperating pair of matched fuses 21, 24 and 51, 54 on the source side of the load transformers 64, fuses 65 being eliminated. For' convenience, fuse 65 in FIG. 3 can be considered as representing one of the pairs of matched fuses 21, 24 and 51, 54 when the present invention is practiced.

Proceeding from the fuse 66 toward the generator, there is found an increasingly larger main fuse 67. The

fuses grow increasingly larger in progressing from fuse toward the generator. If desired, an overload circuit breaker 69 may be provided between the generator and the entire system.

Sizing of the fuses is' done with a chart or set of charts represented in an idealized composite fashion by FIG. 4.

As mentioned above, one of the advantages of the present fusing system over prior art fusing arrangements is the reduction in sizes of the various branch fuses such' as 66,- 67. The reason for this is best seen by' reference to" the following explanation in light of FIG. 4 bearing in mind that the present invention permits sizing all branchprotecting fuses 66 and the main fuse 67 using the load side protecting fuse 24, 54 as the base on which to determine the branch fuse sizes. This is in contrast with the prior art practice of sizing fuses 66, 67 using the source side protecting fuse 21, 51 as the base from which other fuses are sized. The present invention employs fuses 24, 54 as preferably one size smaller than the fuses 21, 51.

Practicing the present invention, the curve X of FIG. 4 represents the load side protecting fuse 24, 54. Using this chart in the manner of one skilled in the art, fuse 66 when sized relative to curve X would be the type of fuse designated by curve A since it would not melt under fault current value 61. Were the prior art practice employed in this instance, curve Y of FIG. 4 representing the source side protecting fuse 21, 51 would be used as the basis for selecting fuse 66, and would result in the selection of a fuse having a time-current characteristic that would not melt at current 61 represented by curve B in FIG. 4. It is evident from the comparison of the two sizing techniques that the present invention permits the employment of fuses having smaller sizes, all other conditions being equal. This allows more steps of fusing and hence improved sectionalizing of the system involved, and smaller and less costly ratings of breaker equipment 69.-

Although the invention has been described with respect to specific embodiments having reference to particular theories of operation, it is not intended to restrict the invention thereto but instead to encompass those modifications, changes, substitutions of equivalents and the like which would be obvious to one skilled in the art. The hypotheses of operation presented above are intended not as a limitation of the invention but as a stepping stone toward promoting the understanding thereof and it is not intended to predicate the invention upon the particular hypotheses involved but rather upon the means devised for arriving at the objects, features, and advantages secured in the practice of this invention.

I claim:

1. The combination with the primary coil of a transformer, disposed within an oil filled casing, said casing being sealed, of electrical fusing means, comprising a first fuse assembly connected in series with said coil and being sealed within the transformer casing so that it is inaccessible whereby replacement thereof is achieved by opening the casing to enable access to said first fuse assembly;

a second fuse assembly connected in series with said coil and said first fuse, said second fuse assembly being disposed within said transformer casing having means for removing said second fuse assembly from outside the casing,

said first fuse assembly having a predetermined timecurrent characteristic, said second fuse assembly having a predetermined time-current characteristic which intersects the time-current characteristic of said first fuse assembly at a predetermined current value, whereby at current values below the predetermined value only the second fuse assembly will blow and at current values above the predetermined value both fuse assemblies will blow.

2. The combination with the source side coil in an oil-cooled transformer having a sealed casing surrounding a body of oil and said coil, of electrical fusing means for said transformer, comprising a first fuse assembly connected in series with said coil and being immersed in the body of oil in said transformer, and being normally inaccessible from outside the casing whereby replacement thereof is achieved by disassembling the sealed casing to enable access to said first fuse assembly, said first fuse assembly having a predetermined time-current characteristic, and

a second fuse assembly connected within the transformer casing in series with said coil and said first fuse 9 10 assembly, having outside the se led casing a means above said predetermined value to isolate the transfor removing said second fuse assembly from within formerthe sealed casing, 4. The combination of a transformer and a fusing syssaid second fuse assembly having a predetermined teIn, comprising a transformer having a Primary coil time-current characteristic; mersed in a sealed, oil filled casing said improved elecsaid first fuse assembly being sized relative to said sectfical fuse system including 0nd fuse assembly so that both undergo melting a first fuse assembly having a fuse element of a P and arcing but the latter cle r fi t i response to determined size connected in series with said coil flow of a current greater than a predetermined value and being normally inaccessible from Outside the but less than that flow required for said first fuse sealed casing whereby replacement thereof is assembly to clear. achieved by disasssembling the casing to enable ac- 3. The combination with the source side coil in an oilcess to said first fuse assembly, and cooled transformer having a casing di a b a second fuse assembly that includes a fuse element of of oil and said coil, said casing being sealed, of electrical a smaller size than said first fuse clement connected fuse means comprising within the sealed casing in series with said coil and a first fuse assembly connected in series with said coil said first fuse element, and further ineilldes Outside and being immersed in the body of oil in said transthe sealed casing a means for removing said second former, said first fuse assembly having predetermined fuse element from Within the sealed casing, clearing and lti ti h t i ti d said second fuse assembly fuse element having a cleara second fuse assembly connected with the transg tiIne greater than the melting time of said first former casing in series with said coil and said first fuse assembiy fuse element current in excess of fuse, said secondfuse assembly having predetera predetermined currentmined clearing and melting time characteristics, said first fuse assembly being sized relative to said References Cited by the Exammel' second fuse assembly so that the melting time char- UNITED STATES PATENTS acteristic of said first fuse assembly intersecting the 2,453,688 1 4 Yonkers 200-123 XR clearing time characteristic of the second fuse as- 2,809,254 10/57 Edsau 200 123 XR sembly at a predetermined current value, whereby 3,014,158 12/61 Nelson et a1. sa1d second fuse assembly blows under all overload current conditions and said first fuse assembly blows SAMUEL BERNSTEIN, Primary Examiner. 

1. THE COMBINATION WITH TE PRIMARY COIL OF TRANSFORMER, DISPOSED WITHIN AN OIL FILLED CASING, SAID CASING BEING SEALED, OF ELECTRICAL FUSING MEANS, COMPRISING A FIRST FUSE ASSEMBLY CONNECTED IN SERIES WITH SAID COIL AND BEING SEALED WITHIN THE TRANSFORMER CASING SO THAT IT IS INACCESSIBLE WHEREBY REPLACEMENT THEREOF IS ACHIEVED BY OPENING THE CASING TO ENABLE ACCESS TO SAID FIRST FUSE ASSEMBLY; A SECOND FUSE ASSEMBLY CONNECTED IN SERIES WITH SAID COIL AND SAID FIRST FUSE, SAID SEOND FUSE ASSEMBLY BEING DISPOSED WITHIN SAID TRANSFORMER CASING HAVING MEANS FOR REMOVING SAID SECOND FUSE ASSEMBLY FORM OUTSIDE THE CASING, 